In this talk I review a covariant generalization of Einstein's general relativity which allows the existence of a term proportional to T_{\alpha\beta} T^{\alpha\beta} in the action functional of the theory ( T_{\alpha\beta} is the energy-momentum tensor). This simple generalization leads to interesting consequences in the early universe. Applying this theory to a homogeneous and isotropic spacetime, we find that there is a maximum energy density ρ_{max}, and correspondingly a minimum length a_{min}, in the thermal history of the universe. This means that there is a bounce at early times, and this theory avoids the existence of an early-time singularity. Also I review the possible consequences of this theory in the dynamics of compact objects like Neutron stars. More specifically I discuss the mass-radius relation for Neutron stars in this model.

The word "quantization" is used both in physics and mathematics in many different senses. The common basis of all these theories is that the classical and quantum mechanics are just different realizations of the same abstract scheme. Geometric quantization goal is the construction of quantum objects using the geometry of the corresponding classical objects as a point of departure. The geometric quantization procedure falls into the following three steps: prequantization, polarization and metaplectic correction. Prequantization produces a natural Hilbert space together with a quantization procedure for observables that exactly transforms Poisson brackets on the classical side into commutators on the quantum side. Nevertheless, the prequantum Hilbert space is generally understood to be "too big". The idea is that one should then select a Poisson commuting set of n-variables on the 2n-dimensional phase space and consider functions that depend only on these n variables. The n variables can be either real-valued, resulting in a position-style Hilbert space, or complex valued. A polarization is a coordinate independent description on such a choice of n Poisson-commuting functions. The metaplectic correction is a technical modification of the above procedure that is necessary in the case of real polarization and often convenient for complex polarization.

The inflationary models solve several problems of cosmology, such as horizon problem and the flatness of the Universe. But however there are lots of inflationary models which Cosmic Microwave Background (CMB) can't rule out them now. Non-Gaussianity and tensor perturbation are two complementary probes for inflationary models. This is because the inflationary models have different predictions for these two observables, however they are not detected yet. The deviation of the scale invariance of the primordial power spectrum is another opportunity which is detected by CMB data but with low statistical significance. In this presentation I will talk about the last opportunity and justify how Large Scale Structure will help us to rule out more inflationary models or the inflation scenario itself (through some idea about clock signal models which is developed recently).

Strong gravitational lensing systems contain a wealth of information that can be used to estimate the expansion history of the universe. The light rays from different images of a source quasar experience different gravitational potentials and optical paths which result a time delay in the associated light curves. A reliable estimation of the time delay has crucial role in the process of expansion history estimation. In this talk we describe our proposed algorithm for time delay estimation which shows outstanding results in the Time Delay Challenge (TDC). This algorithm consists of the smoothing and cross-correlation methodologies as well as a quick way for error estimation. In continue we explain the modified version of this algorithm which recently we have developed. In this version we introduce a new time delay estimator which includes weighted cross-correlation and some other changes in the procedure of estimation. The modified algorithm shows more precise results on TDC simulated data. At the end we discuss the estimation of time delay related to the light curves of the lensed quasar SDSS J1001+5027 system and compare our results with the time delays obtained by different group of researchers for the same system.

In this talk I will explain the possibility of primordial black holes (PBH) formation from particle production during inflation. We consider the scalar and the gauge quanta production in inflation models, where in the latter case, we focus in two sectors: inflaton coupled i) directly and ii) gravitationally to a U(1) gauge field. We do not assume any specific potential for the inflaton field. Hence, in the gauge production case, in a model independent way we show that the non-production of DM PBHs puts stronger upper bound on the particle production parameter. Our analysis show that this bound is more stringent than the bounds from the bispectrum and the tensor-to-scalar ratio derived by gauge production in these models. In the scenario where the inflaton field coupled to a scalar field, we put an upper bound on the amplitude of the generated scalar power spectrum by non-production of PBHs. As a by-product we also show that the required scalar power spectrum for PBHs formation is lower when the density perturbations are non-Gaussian in comparison to the Gaussian density perturbations.

We will consider what is perhaps the simplest model for the formation of non-linear (gravitationally bound) structures. Imagine that, we have a spherical region with density higher than the density of the background (which is equal to the critical density). According to general relativity, the evolution of this spherical overdensity will be independent of the background evolution, so that it will evolve exactly like a sub-universe with density higher than the critical density.

We study the evolution of the "non-perturbative" metric perturbations in a Bianchi background in the long-wavelength limit. By applying the gradient expansion to the equations of motion we exhibit a generalized "Separate Universe" approach to the cosmological perturbation theory. Having found this consistent separate universe picture, we introduce the δM formalism for calculating the evolution of the tensor perturbations in anisotropic inflation models in almost similar way as the so-called δN formula for the superhorizon dynamics of the curvature perturbations. Likewise its ancestor, δN formalism, this new method can substantially reduce the amount of calculations related to the evolution of the tensor modes.

The evolution of galaxies is driven by the birth and death of stars. Nearby galaxies in the Local Group offer us a complete suite of galactic environments that is perfect for studying the connection between stellar populations and galaxy evolution. Therefore, the Local Group of galaxies offers a superb near-field cosmology site. Here, we can reconstruct the formation histories, and probe the structure and dynamics, of spiral galaxies as well as the many dwarf satellite galaxies surrounding the Milky Way and Andromeda, and of isolated dwarfs using the resolved stellar populations. In this talk, first I will introduce a near-IR monitoring campaign of the Local Group spiral galaxy M33, carried out with the UK IR Telescope (UKIRT). The pulsating evolved stars are identified and their distributions are used to derive the star formation rate as a function of age. I will outline the methodology and will present the results for the central square kiloparsec and for the disc of M33. Then, I will introduce an ongoing survey (started in June 2015) to monitor the Local Group dwarf galaxies accessible from La Palma with Isaac Newton Telescope (INT). We will use the luminosity distribution of the LPVs to reconstruct the star formation history – a method we have successfully applied in other Local Group galaxies.

On the origin of morphological differences in the powerful radio galaxies

حلیمه میرآقایی (پژوهشکده نجوم پژوهشگاه دانش های بنیادی - IPM)

Abstract:

Powerful radio galaxies display a very wide range of properties, both in their extended radio morphologies and in their optical spectra. We study the environment and host galaxy properties of radio galaxies with different radio morphologies from compact sources to very extended double lobed radio galaxies and with different optical spectra as high excitation (HERG; quasar-mode) and low excitation (LERG; jet-mode) radio galaxies in order to separate and distinguish the factors that drive the radio morphological variations from those responsible for the differences in spectral properties. We particularly compare i) compact sources with Fanaroff–Riley (FR) extended radio sources from the same class of excitation, ii) HERGs with the LERGs, iii) FR type I with the FR type II from the same class of excitation. We discuss the results and what causes the differences in each comparison.

Sharif University of Technology - Weekly Cosmology Seminar

دانشکده فیزیک دانشگاه صنعتی شریف

سمینار هفتگی گروه کیهان شناسی

High angular resolution of protoplanetary disks

نرگس جامی (پژوهشکده نجوم پژوهشگاه دانش های بنیادی - IPM)

Abstract:

Planetary systems are born in circumstellar gas and dust disks surrounding Young Stellar Objects (YSOs). To understand how planetary systems form, a detailed knowledge of the structure and evolution of these disks is required. Although, this is almost well understood for the regions of the disks observed with a spatial resolution of several AUs, the structure of these disks probed at a few AU scale and especially inward of 1 AU remains a puzzle. In recent years, it has become possible to directly spatially resolve the inner region of protoplanetary disks with optical interferometry technique, which will be detailed in my talk.

I will review the quantization and effective dynamics of closed FLRW model within loop quantum cosmology, paying special attention to the existence of an ambiguity in the quantization process. This ambiguity arises from different ways of defining the curvature operator. One definition is through closed paths and the other one is through open paths. For spatially flat models these two methods coincide and for spatially non-flat anisotropic models the first one is not admissible. However in the case of the closed FLRW model, one can apply both methods and they yield different quantum theories. In this talk I will show the different effective descriptions they provide of the bouncing cyclic universe. In particular, the most significant difference is that in the theory defined by the 'open path' method, there is not one, but two different bounces through which the cyclic universe alternates. Also, I will show that the quantum corrections of both theories can cure the unboundedness of matter density and expansion in effective phase space.

Convective dynamo action in a spherical shell: symmetries and modulation

Raphaël Raynaud

School of Astronomy - IPM

Abstract:

We consider dynamo action driven by three-dimensional rotating anelastic convection in a spherical shell. Motivated by the behaviour of the solar dynamo, we examine the interaction of hydromagnetic modes with different symmetries and demonstrate how complicated interactions between convection, differential rotation and magnetic fields may lead to modulation of the basic cycle. For some parameters, Type 1 modulation occurs by the transfer of energy between modes of different symmetries with little change in the overall amplitude; for other parameters, the modulation is of Type 2, where the amplitude is significantly affected (leading to grand minima in activity) without significant changes in symmetry. Most importantly, we identify the presence of 'supermodulation' in the solutions, where the activity switches chaotically between Type 1 and Type 2 modulation; this is believed to be an important process in solar activity.

There are many successes regarding the standard model of cosmology but there are also problems with this model. A few of these relate to the existence of a preferred axis in the universe which undermines the assumption of isotropy and homogeneity of the standard model known as the cosmological principle. It is therefore of high importance to utilize probes such as Supernovae Type Ia, which are pretty useful for such an investigations owing to the practical redshift range they are found in. Several different methods have been employed in this field of research so far. In this talk I will go over our latest work in this regard.

In a microlensing event, the light of a background source star is temporarily magnified owing to passing through the gravitational field of a massive foreground object. One of important advantages of microlensing observations is detecting cold and dark extra solar planets often located far from us. For this reason nowadays two survey and several follow up telescopes are probing ongoing microlensing events toward the Galactic centre. Consequently, many astronomers are busy to reduce the data of these events, i.e. converting crude images to light curves and then characterising the lens and source stars. In this talk, I will explain all steps for reducing the data of a microlensing event which are calibrating the images, finding a good reference images, doing difference photometry (DIA) and finally constructing a light curve. Then, I will discuss how to obtain the best fitted model to the resulted light curve through Markov Chain Monte Carlo (MCMC) simulation.

One of the most important studies in cosmology is the study of Large Scale Structure of cosmos, where one of the useful and valuable tools is the Excursion Set Theory( EST). EST or Extended Press-Schechter is a theory based on stochastic processes which gives us the distribution function (mass function) of dark matter halos. In order to have smoothed density field, Excursion Set Theory uses a specific window function (filter) called sharp k-space, which is a top-hat in Fourier space. If one uses this specific window function the process of density field with respect to variance will be a Markov process, with any other window function the process won't be a Markov process.

Excursion Set Theory uses the concept of Markov processes to define some mirror trajectories and that's one of the basics of the theory. However practically, instead of using sharp k-space filter, physicists use more common filters such as top-hat and Gaussian. In the other side there is an important concept in stochastic processes called Einstein-Markov length, which is a length of nature, where a stochastic process becomes Markovian. In this Thesis using Dark-Sky simulation we investigate Einstein-Markov length of density contrast when it is smoothed with a top-hat filter. The results give us the best variance step which gives Markov process for smoothed density contrast with a top-hat window function.

One of the biggest challenges facing cosmologists is the nature of dark matter. The cosmological observations, like the rotation curve of galaxies, gravitational weak Lensing and cosmic microwave background radiation indicate that baryonic matter alone cannot explain the density of matter in the Universe. In the context of standard cosmological models, it means that the Universe is made of an unknown component (Dark Matter), which approximately 27% of total energy-momentum. The Modify Gravity and is an alternative theory that is introduced to describe these observations. In This thesis, we introduce the issue of the measurements of accelerated observers in Minkowski space-time that goes beyond the postulate of locality. This is a step towards introducing nonlocal special theory of relativity. Inertia and gravitation are intimately connected in accordance with the equivalence principle. In order to arrive indirectly at a nonlocal gravity, to explain nature of dark matter, we work within framework of teleparallel gravity. Teleparallel gravity is a gauge theory for the translation group and is an alternative theory, fully equivalent to general relativity that renders field equations of gravity in close analogy with Maxwell's equations. This analogy helps us to present Nonlocal gravity. The linear approximation of this theory is thoroughly examined. It is shown that there is a room for dark matter like dynamics in the linear regime. Finally we introduce the gravitational dipole moment to present phenomenology of nonlocal gravity.

The distance measurement is the corner stone of the modern cosmology. They are measured via "Doppler Effect", accordingly the distances in cosmology are obtained in red-shift space. Despite the cosmological background expansion, and the peculiar velocities due to the local density contrast distort these measurements. One of the main challenges in upcoming cosmological studies is to distinguish the peculiar velocity effect from the background expansion. Supernova Type Ia as standard candles can be think of a crucial observation to address the above problem. In this thesis, we study the effect of cosmological perturbations on distance-redshift relation of the SNIa. The corrective effects are from weak gravitational lensing, integrated Sachs Wolfe effect and the peculiar velocity of the SNe Ia host galaxies. Understanding these corrections will help us to disentangle the peculiar velocity effect from the background dynamic and other corrections. We conclude that by future surveys where the number of SNIa becomes larger and the precision will be improved, we will be able to find the peculiar velocity field and also the bulk flow.

Helioseismology and Helioseismic Inversion Helioseismology studies the structure and dynamics of the Sun's interior by observing oscillations on the surface. In recent years, helioseismology has made substantial progress towards the understanding of the physics of solar oscillations and the physical processes inside the Sun. In addition, a new field called local helioseismology has been developed which studies oscillation travel times and its aim is to address fundamental questions in time-distance helioseismology, such as the choice of travel-time measurements and the sensitivity kernels for inversions, to probe the complex structures like convective flows. The choice of regularization method and considering computational cost in terms of number of observations are key factors in helioseismic inversions.

Galaxies cluster under the influence of their mutual gravitation and the characterization of this clustering problem is a subject of great interest as it somehow helps in understanding the structure formation of the universe. In the last two decades many approaches have been developed in understanding the LSS of the universe from galaxies to the cosmic web like percolation, topology, distribution functions, voids, correlation functions and fractals. All of them have been found useful in studying the clustering of galaxies problem. We have developed the theory on the basis of statistical mechanics and thermodynamics and have found it a convenient method for describing various statistical properties of galaxy clusters in an expanding universe. Statistical mechanical treatment of the cosmological many body problem in combination with the thermodynamics explains the behaviour of entropy and very recently we have postulated that how a kind of cosmological phase transition can also be described for galaxy clusters. The analytical method developed in our theory has been extended from point mass model to extended mass model of galaxy clusters. We also study various dynamical properties of galaxy clusters. In my talk, I will discuss the development of our theoretical model.

Abstract: Whilst often phrased in terms of accelerating expansion of the Universe or discovering the nature of the dark energy, maybe the first thing that we can think about, is measuring distances or the geometric observations. The Baryon Acoustic Oscillations (BAO) are the remaining relic of the baryon and photon decoupling epoch. And offer a powerful method of measuring cosmological distances as a standard ruler. In this thesis we first review the theoretical, observational and statistical aspects of the BAO as a statistical standard ruler and examine the impact BAO will have on our understanding of dark energy. Then we calculate this scale by LRGs data and constraints on cosmological parameters. Then we introduce the error factors like Weak Gravitational Lensing, Redshift Space Distortions, bias and non-linearities and calculate the effect of these factors on BAO scale. Additionally we combine these effects and check that what happened on the Baryon Acoustic Oscillations scale. After that we introduce the Linear Point on the two point correlation function and estimate the effect of Weak Gravitational Lensing on this point.

سمینارهای هفتگی گروه چگال ماده نرم

Fabrication and Functionalization of magnetic microswimmers named as MagnetoSperm for potential biomedical application

Fatemeh Asadi

Department of Physics, Shahid Beheshti University

زمان: شنبه ۴ اردیبهشت ۹۵

ساعت ۱۵:۳۰

مکان: آمفی تئاتر دانشکده‌ی فیزیک (تالار جناب)

By Portal Administrator
4/21/16
83107 Views

سمینار مشترک گروه ذرات و کیهان شناسی

سمینار مشترک گروه ذرات و کیهان شناسی

Primordial Inflationary Universe

حسن فیروزجاهی

(پژوهشکده نجوم پژوهشگاه دانش های بنیادی - IPM)

لطفا به مکان و زمان سمینار توجه نمایید.

سه شنبه 7 اردیهشت ماه 1395، ساعت 15:00

دانشکده فیزیک، آمفی تئاتر دانشکده فیزیک (تالار جناب)

By Portal Administrator
4/21/16
83110 Views

سمینارهای هفتگی فیزیک انرژِی بالا

سمینارهای هفتگی فیزیک انرژِی بالا (ذرات بنیادی)

Phenomenology of a new 750 GeV resonance

Dr Sara Khatibi

IPM

Abstract

Recently, ATLAS and CMS experiments reported an excess in the high energy diphoton mass spectrum around 750 GeV with local significance of about 3.6 σ and 2.6 σ, respectively. If this resonance would be confirmed with next year LHC data, it will be a promising sign of the new physics beyond the Standard Model. In this talk, first I will review the CMS and ATLAS reports and then talk about some phenomenological aspects of this new bump.

سه شنبه 31 فروردین ساعت 15:00

طبقه پنجم دانشکده فیزیک اتاق 517

By Portal Administrator
4/16/16
84714 Views

سمینارهای هفتگی گروه چگال ماده نرم

سمینارهای هفتگی گروه چگال ماده نرم

Effect of physico- chemical properties of nanoparticles on kinetic of Amyloid βfibrillation process

Dr. Somayeh Mirsadeghi

Assistant professor in Endocrinology and Metabolism Research Institute,

Tehran University of Medical science

شنبه28فروردین ماه 1395، ساعت 15:30

دانشکده فیزیک، آمفی تئاتر مرکزی (تالار جناب)

By Portal Administrator
4/15/16
84753 Views

سمینارهای هفتگی گروه کیهان شناسی

سمینارهای هفتگی گروه کیهان شناسی

Power asymmetry of the cosmic microwave background (CMB)

in the presence of a massive defect

طاهره رستمی

پژوهشکده نجوم پژوهشگاه دانش های بنیادی (IPM)

Abstract

In a quest to explain the observed anomalous power asymmetry of the cosmic microwave background (CMB), we show that in a model of inflation, broken homogeneity by a point mass can give a picture to understand the origin of dipole asymmetry. In more detail, I will show that the de-Sitter-Schwarzschild (dS-Sch) space-time, in which homogeneity is broken, the power spectrum of curvature perturbations are anisotropic with non-trivial scale-dependent dipole, quadruple and higher multipoles. Also, the primordial power spectrum of tensor perturbations is modified such that it contain anisotropic scale dependent terms.

سمینارهای هفتگی گروه کیهان شناسی

Preparing for Gaia, towards the first Three-dimensional Dust Map in the Milky Way

سارا رضایی خوشبخت

(Max Plank Institute for Astronomy (MPIA), Heidelberg, Germany)

Abstract

We are developing a non-parametric model to reconstruct the three-dimensional (3D) distribution of dust in the Milky Way. Our approach uses observed line-of-sight (l.o.s) extinctions towards stars at different positions in the Galaxy. These give the integrated dust density along each l.o.s. Making weak assumptions about the correlation of the dust, we infer the most probable 3D distribution of dust which explains the observed extinctions, even at arbitrary points. Given distances and extinctions estimated from the Gaia photometry and astrometry for tens of millions of stars, we plan to build a detailed map of dust in our Galaxy.

سمینارهای هفتگی گروه کیهان شناسی

سمینارهای هفتگی گروه کیهان شناسی

Jets and outflows: Theory, Observation, Simulations

سمیه شیخ نظامی

(پژوهشکده نجوم پژوهشگاه دانش های بنیادی - IPM )

Abstract

Jets are powerful signatures of astrophysical activity and are observed over a wide range of luminosity and spatial scale. Typical jet sources are young stellar objects (YSOs), micro-quasars, and active galactic nuclei (AGNs), while there are indications of jet motion also for a few pulsars and for gamma-ray bursts. In the first part of the talk, I give a brief introduction on astrophysical jets, their formation and acceleration. In the next part of the talk, I present our recent results of the first ever three-dimensional (3D) magneto-hydrodynamic (MHD) simulations of the accretion–ejection structure. The 3D evolution of jets launched symmetrically from single stars but also jets from warped disks in binary systems have been studied. Various model setups have been applied and tested by simulating a stable and bipolar symmetric 3D structure from a single star–disk–jet system. Our simulations show typical 3D deviations from axial symmetry, such as jet bending outside the Roche lobe or spiral arms forming in the accretion disk.

سمینارهای هفتگی فیزیک انرژِی بالا

سمینارهای هفتگی فیزیک انرژِی بالا (ذرات بنیادی)

CMS Results in 13 TeV

Saeid Paktinat

Abstract: In 2015, LHC collided protons in the unprecedented center of mass energy of 13 TeV. The CMS experiment has analysed the collision data in many different aspects. Some important analyses are reviewed in this talk.

Gravity waves predicted by the theory of general relativity about 100 years ago. The strength of the gravity wave is characterized by the fractional changes in the lengths of two perpendicular axes which were too weak for detection till now. Recent progress enhanced the sensitivity of detectors for observing gravity waves. This talk covers some of the progress on this subject and specially discuss about the physics of LIGO detector.

سمینارهای هفتگی گروه چگال ماده نرم

سمینارهای هفتگی گروه چگال ماده نرم

Modeling and simulation of actin and its network

Tiam Heydari

Department of physics, Sharif University of Technology

The protein actin is a part of the cytoskeleton and, therefore,responsible for the mechanical transmission of forces and interactions include the conversion of mechanical stimuli to biochemical signals. In this review, we focus solely on the continuum mechanical derivation of mechanical models. Furthermore, we consider solely mechanical modeling leaving out important topics of current actin research.

شنبه1اسفند ماه 1394، ساعت 15:30

دانشکده فیزیک، آمفی تئاتر مرکزی (تالار جناب)

By Portal Administrator
2/17/16
97449 Views

سمینارهای هفتگی گروه کیهان شناسی

سمینارهای هفتگی گروه کیهان شناسی

کسر دوتایی ها و تفکیک جرمی در خوشه ی باز آلفا برساووش

The binary fraction and mass segregation in Alpha Persei open cluster

نجمه شیخی (دانشکده فیزیک دانشگاه تحصیلات تکمیلی زنجان )

Abstract:We have obtained membership probabilities of stars within a field of :3ofrom the center of the open cluster Alpha Persei using proper motions and photometry from the PPMXL and Wide-field Infrared Survey Explorer catalogues. We have identified 810 possible stellar members of Alpha Persei. We derived the global and radial present day mass function of the cluster and found that they are well matched by two stage power law relations with different slopes at different radii. The global MF of Alpha Persei shows a turnover at 0.62Me with low and high mass slopes of αlow =0.50 ±0.09 and αhigh = 2.23 ±0.14, respectively. The high mass slope of the cluster increases from 2.01 inside 1o.10 to 2.63 outside 2o .2, whereas the mean stellar mass decreases from 0.95 to 0.57Mein the same regions, signifying clear evidence of mass segregation in the cluster. From performing a series of Monte Carlo simulations, we obtained a binary fraction of fbin= 34±12 per cent for stars with 0.70<m/Me<4.68. This is significantly larger than the observed binary fraction, indicating that this open cluster contains a large population of unresolved binaries. Finally, we corrected the MF slopes for the effect of unresolved binaries and found low and high mass slopes of αlow= 0.89 ± 0.11 and αhigh = 2.37 ± 0.09 and a total cluster mass of 352Me for Alpha Persei.

سمینارهای هفتگی گروه کیهان شناسی

سمینارهای هفتگی گروه کیهان شناسی

Lensing as a probe of early universe physics

فربد حسنی (دانشکده فیزیک دانشگاه صنعتی شریف )

Abstract:The standard cosmological model LCDM describes the large-scale structure and early universe greatly well. However some hints of departures from LCDM exist in the present cosmological data in large scales as well as small scales. Of these, significant attention has been paid to various "anomalies" in measurements of the cosmic microwave background radiation. Observations of Planck collaboration indicate the persistence of the dipole modulation in the primordial power spectrum which first seen in WMAP data and it is found with local variance estimator. In the case, if the anomalies are not the statistical flukes It is worthwhile considering whether observations other than CMB might be able to address this question. In our recent work arXiv:1511.05534, we propose that the CMB lensing and also CMB lensing cross-correlation with galaxy lensing can be used as a unique probe for detection of these anomalies especially dipole modulation and constraining on their scale dependence.

In the first part of my talk I will review the CMB lensing and galaxy lensing theories. Then I will show how these observations would help us to probe the early universe physics.

Seminar 1- Abstract:Over and over again, new observations come in that match the predictions of ΛCDM. But there are still a few anomalies that bug us, especially on relatively small (galaxy-sized) scales. One such anomaly is the "too big to fail" problem. In this talk, first, I will explain what this problem exactly is and then I will briefly review some solutions have been found to explain it so far.

Seminar 2-Abstract:The nature and the properties of Dark Energy are among the most significant unsolved problems in physics. We now know that Dark Energy accounts for about 70% of the energy density of the Universe and is causing the cosmic expansion to accelerate. But there exist only very few observational windows into the properties of this phenomenon. Measurements of the Integrated Sachs-Wolfe effect (ISW) provide a powerful method to probe Dark Energy, as this effect is sensitive to the time evolution of the gravitational potential sourced by Large Scale Structure. One interesting feature of several of the previous studies is that the cross correlation signal lies above the predicted value in the standard cosmological model (in which the Dark Energy is a Cosmological Constant). We could show a dark energy with redshift dependent equation of state ( w_0 + w_a*(1/(1+z)) ) is a good fit to data, but if we change our assumption about a "constant bias" and use a redshift dependent bias LambdaCDM is still a good fit too! Here is the question: Redshift dependent bias or varying with time equation of state. We think weak lensing can break this degeneracy!

سمینارهای هفتگی گروه کیهان شناسی

Seminar 1: The Effective Field Theory of Cosmological Large Scale Structures

نیما چرتاب (دانشکده فیزیک دانشگاه صنعتی شریف )

Seminar 2: Halo Merger Trees and Halo Assembly Time

محمدرضا ایرملو (دانشکده فیزیک دانشگاه صنعتی شریف )

Seminar 1- Abstract:Over the next decade, large scale structure (LSS) surveys will play an increasingly important role in the measurement of cosmological parameters and as a probe of initial conditions. The Effective Field Theory of LSS is an efficient approach to make precise analytical predictions on large scale structures. In this talk, I will explain how all of the complicated dynamics at the scales of galaxies and clusters could affect large scale matter perturbations. After reviewing the recently developed effective field theory of large scale structure, I will compare the matter power spectrum measured from the simulations with theoretical models (e.g. EFT of LSS and Halo Model).

Seminar 2-Abstract:A powerful method for computing the mass function of dark matter halos is provided by Excursion Set Theory (EST), where density perturbations evolve stochastically with the smoothing scale, and the problem of computing the probability of halo formation is mapped into the so-called first passage time problem in the presence of a barrier. While the full dynamical complexity of halo formation can only be revealed through N-body simulations, Excursion Set Theory provides a simple analytic framework for understanding various aspects of this complex process. With EST one can calculate Progenitor Mass Function, Plant Halo Merger Trees, Find Halo Assembly Time, Calculate the Bias and lots of other things. In this talk I will present some methods of Halo Merger Trees, Halo Assembly Time and Bias.

سمینارهای هفتگی گروه چگال ماده نرم

Theoretical prediction of equilibrium yield for complex self-assembly based on connectivity graph

Keyvan Kouroush

Departmant of chemistry , Sharif university of Technology

شنبه 26 دی ماه 1394، ساعت 15:30

دانشکده فیزیک، آمفی تئاتر مرکزی (تالار جناب)

By Portal Administrator
1/11/16
102752 Views

سمینارهای هفتگی گروه کیهان شناسی

سمینارهای هفتگی گروه کیهان شناسی

Understanding the nature of Barred

galaxies:

From morphology to dynamics

علیرضا مولایی نژاد (پژوهشکده نجوم پژوهشگاه دانش های بنیادی (IPM) )

Abstract:Barred galaxies represent a considerable fraction of the entire disc galaxy population. Bars could be considered as the key drivers of the internal secular evolution in disc galaxies by redistribution of the angular momentum and triggering of star formation, or changing the morphological structure of disk galaxies. In edge-on or highly inclined systems, bars are most easily recognized by boxy and peanut-shapes, and sometimes X-shape morphology formed by the stellar material above the disc plane. BP bulges specifically and bars in general produce distinct kinematic features that can be easily detected in galaxies. I will present our recent results of the study on the stellar kinematics and populations of a sample of intermediate inclined disk galaxies, obtained with the SAURON integral-field spectrograph on the 4.2m William Herschel Telescope. I will also discuss our approach to unveil hidden bars and inner disks on the basis of their kinematics and introduce a method to quantify the cylindrical rotation, as a common kinematics feature of barred galaxies. The results of this study allow us to establish the link between the stellar kinematics and populations of bulges and bars with their surrounding thin and thick disks, and relate them to the properties observed in disk galaxies.

سمینارویژه گروه کیهان شناسی

سمینارویژه گروه کیهان شناسی

Finding dwarf galaxies with small telescopes

بهنام جوانمردی

(Argelander-Institute for Astronomy-University of Bonn)

لطفا به مکان و زمان نامتعارف سمینار توجه نمایید.

Abstract:

In the framework of the standard model of cosmology, galaxies form in a hierarchical process. In this scenario, many substructures (or satellites) are expected to exist around massive (Milky Way-like) galaxies. The number, spatial distribution, and other properties of these satellite galaxies can be used to test different models of structure formation. Therefore, increasing the number of known dwarf satellite galaxies outside the Local Group is very crucial. We recently launched the Dwarf Galaxy Survey with Amateur Telescopes (DGSAT) project to use the ability of small telescopes for finding low surface brightness galaxies. In this talk, I will briefly highlight the importance of finding dwarf satellite galaxies and will present the first results of the DGSAT project (http://arxiv.org/abs/1511.04446) which is the discovery of 11 low surface brightness systems in the field of 6 nearby spiral galaxies.

شنبه 12 دی ماه 1394، ساعت 11:00

دانشکده فیزیک، طبقه پنجم–اتاق 517

By Portal Administrator
12/30/15
104139 Views

سمینارهای هفتگی گروه چگال ماده نرم

سمینارهای هفتگی گروه چگال ماده نرم

Physics of Crumpling

Dr Mehdi Habibi

Departmant of Physics and Astronomy

University of Amsterdam , Netherland

شنبه 12 دی ماه 1394، ساعت 15:30

دانشکده فیزیک، آمفی تئاتر مرکزی (تالار جناب)

By Portal Administrator
12/30/15
104163 Views

سمینارهای هفتگی گروه کیهان شناسی

سمینارهای هفتگی گروه کیهان شناسی

Dark Energy at Large Scales: Homogeneous or what?

شهرام خسروی (دانشکده فیزیک دانشگاه خوارزمی )

Abstract:The Integrated Sachs Wolfe (ISW) cross correlation with the galaxy distribution in late time is a promising tool to constrain the dark energy properties. In this work we study the effect of dark energy clustering on the ISW-galaxy cross correlation. Indicating the fact that the bias parameter between the distribution of the galaxies and the underlying dark matter introduce a degeneracy and complications. We argue that as the time of the galaxy's host halo formation is different from the observation time, we have to consider the evolution of the halo bias parameter. We indicate that any deviation from ΛCDM model will change the evolution of the bias as well. Also we show that the halo bias strongly depends on the sub-sample of galaxies which is chosen for cross correlation. We show that joint kernel of ISW effect and the galaxy distribution have the dominant effect on the observed signal, accordingly we can enhance the signal of a specific dark energy model by choosing an appropriate tracer. More specifically we compare the clustered dark energy models with two samples of galaxies. First is a sub-sample of galaxies from Sloan Digital chosen with the r-band magnitude 18<r<21 with a host dark matter halos of mass M∼1012M⊙ and formation redshift of z∼2.5. Secondly with the sub-sample of Luminous Red galaxies with a host dark matter halos of mass M∼1013M⊙ and formation redshift of z∼2.0. Using the evolved bias we improve the χ2 for the ΛCDM which it reconcile the ∼1-2σ tension of the ISW-galaxy signal and ΛCDM prediction. Finally we show how sub-samples change the bias parameter and will improve the constrains on dark energy clustering.

سمینار مشترک گروه ذرات و کیهان شناسی

سمینار مشترک گروه ذرات و کیهان شناسی

The generation of CMB B-mode polarization due to Compton

scattering and any new interaction such as

CMB -CNB interactions

روح اله محمدی (موزه علوم و فناوری ایران )

Abstract: By approximately solving the first order of Quantum Boltzmann Equation for the density matrix of a photon ensemble, and time-evolution of Stokes parameters, we briefly show that how to add any new interaction in right side of Boltzmann equation. The magnetic-type of linear polarization, B-mode polarization, for the Cosmic Microwave Background cannot be generated by Compton scattering in the case of scalar mode of matter perturbations but this is possible to have B-mode in framework of the Compton scattering in the case of tensor mode of matter perturbations. For this reason, the ratio of tensor to scalar mode of matter perturbation, r-parameter, is estimated (at least for small $l$) by comparing the B-mode power spectrum with E-mode one $r\sim C_l^{BB}/C_l^{EE}$. We study the polarization of CMB specially B-mode polarization due to the weak interactions of Cosmic Neutrino Background (CNB) and CMB in additional to Compton scattering in both cases of scalar and tensor metric perturbations. In this framework, we show that B-mode polarization power spectrum $C_l^{BB}$ receives some contributions from the scalar and tensor modes which have significant effects on the value of r-parameter and also we show that B-mode polarization power spectrum can be used as indirectly detector for CNB which is very difficult to detect by using usual neutrino detectors.

سمینارهای هفتگی گروه کیهان شناسی

We present $N$-body simulations of globular clusters, exploring the effect of different galactic potentials on cluster sizes, $r_h$. For various galactocentric distances, $R_G$, we assess how cluster sizes change when we vary the virial mass and concentration of the host galaxy's dark-matter halo. We show that sizes of GCs are determined by the local galactic mass density rather than the virial mass of the host galaxy. We find that clusters evolving in the inner haloes of less concentrated galaxies are significantly more extended than those evolving in more concentrated ones, while the sizes of those orbiting in the outer halo are almost independent of concentration. Adding a baryonic component to our galaxy models does not change these results much, since its effect is only significant in the very inner halo. Our simulations suggest that there is a relation between $r_h$ and $R_G$, which systematically depends on the physical parameters of the halo. Hence, observing such relations in individual galaxies can put a new observational constraint on dark-matter halo characteristics. However, by varying the halo mass in a wide range of $10^{9}\leq M_{vir}/\msun \leq10^{13}$, we find that the $r_h-R_G$ relationship will be nearly independent of halo mass, if one assumes $M_{vir}$ and $c_{vir}$ as two correlated parameters, as is suggested by cosmological simulations.

Abstract:We extend the investigation of the long mode effects to other large scale observables most importantly galaxy number counts and their observed power spectrum. When defined correctly, genuine observables should not only be gauge invariant but also devoid of any gauge artifacts. One such gauge artifact is a pure gradient mode. We confirm that a long gradient mode which is still outside observer's horizon leaves no imprint on the large scale observables at first order. We also confirm that the galaxy number counts are not affected by these modes.

سمینار مشترک گروه ذرات و کیهان شناسی

(Large Scale Structure of the Universe: An Effective Field Theory Approach)

علی اکبر ابولحسنی (دانشگاه صنعتی شریف)

Abstract: A perturbative description of Large Scale Structure is a cornerstone of our understanding of the observed distribution of matter in the Universe. Renormalization is an essential and defining step to make this description physical and predictive. In this talk I will start off by a brief review of preliminaries of standard perturbation theory (SPT) and diagrammatic representation of perturbation theory. Next I will try to elaborate the very meaning of "renormalization" in the Effective Field Theory of Large Scale Structure (EFT of LSS) and the structure of the so-called counter-terms. In closing I will show the results of EFT of LSS have been very encouraging which has a percent level agreement with the numerical dat

Circumstellar disks around the Galactic bulge stars are un-resolvable from their host stars and make a net polarization signal due to their projected elliptical shapes. Gravitational microlensing can magnify these signals and make them be resolved. The main aim of this project is to determine what extra information about these disks can be extracted from polarimetry observations of microlensing events in addition to those given by photometry ones. Hot disks which are closer to their host stars are more likely to be detected by microlensing, owing to more contributions in the total flux. Although the polarimetric efficiency for detecting this kind of disks is similar to the photometric observation, but polarimetry observations can help to constraint the disk geometrical parameters e.g. the disk inner radius and the lens trajectory with respect to the disk semimajor axis. On the other hand, the time scale of polarimetric curves of these microlensing events generally increases while their photometric time scale does not change. Monte Carlo simulations also show that almost 4 optically-thin disks around the Galactic bulge sources are detected (or even characterized) through photometry (or polarimetry) observations of high-magnification microlensing events during 10 years monitoring of 150 million objects toward the Galactic bulge.

یکشنبه 24 آبان ماه 1394، ساعت 15:00

دانشکده فیزیک، طبقه اول کلاس فیزیک 4

By Portal Administrator
11/11/15
111063 Views

سمینا رگروه چگال ماده نرم

سمینا رگروه چگال ماده نرم

Force field parameterization of biomolecules

Ehsan Nikbin

Department of Physics, Sharif University of Technology

شنبه 23 آبان ماه 1394، ساعت 15:30

آمفی تئاتر مرکزی دانشکده (تالارجناب)

By Portal Administrator
11/11/15
111064 Views

سمینارهای هفتگی گروه کیهان شناسی

سمینارهای هفتگی گروه کیهان شناسی

Searching for DIB origin with VLT; EDIBLES large survey

امین فرهنگ (پژوهشکده نجوم پژوهشگاه دانش های بنیادی - IPM )

Abstract:

Observing the first DIBs, in the spectra of reddened stars, by Heger (1922) was the starting point to the oldest puzzle in stellar spectroscopy that still remains unresolved. At the present, the DIBs are known to comprise 500 narrow to broad interstellar absorption features, which are observed ubiquitously between 4000 and 10000 Å (Herbig 1995). Over the last decade, many astronomical and laboratory experiments have been accomplished to find and understand DIB carriers as well as a number of theoretical studies. There are still ambiguities about DIB carriers. Specific groups of UV-resistant molecules, such as polycyclic aromatic hydrocarbons (PAHs), fullerenes, and carbon chains are commonly the more acceptable candidates for DIB carriers (Herbig 1995). We are doing a large survey with the UVES instrument on the Very Large Telescope (VLT) operated by ESO at Paranal Observatory in Chile, to study the carrier of DIBs. This project is named "ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES)". The goal of our project is to study the mysterious diffuse interstellar bands whose fingerprints are seen imprinted on stellar spectra whose lines-of-sight probe the interstellar medium. In particular the survey will measure at high accuracy the atomic and molecular content of diffuse interstellar clouds detected towards more than 150 stars. From detailed modelling we will attempt to reverse engineer physical-chemical properties of the carriers of these unidentified absorption bands.

Near Earth objects (NEO) are solar system objects revolving around the sun and potentially can strike the earth in near future. These objects are mostly small asteroids, comets, comet debris and solar orbiting satellites. A precise method of orbit determination could help to predict path of a hazardous NEO before it get close to the earth. The standard method of orbit determination is traditionally based on Gauss method which uses three independent observations. Gauss method suffers from an inconsistency where it requires the three independent observations to be close to better approximate the Kepler second law. If closeness is comparable to the observational error, Gauss method losses ability to estimate the plane of the orbit. A good estimation of plane of the orbit needs wide spaced observation which would not stratify the Gauss approximation. We have developed a new method of orbit determination based on conserved quantities. We used linear relation between energy and angular momentum to contribute more than three points and determined more precise orbital elements.

Abstract: Gamma Ray Burst is short lived transients ofγ-ray radiation that is the most energetic explosions in the universe. We discuss the generation of the circular polarization of Gamma Ray Bursts (GRBs) photons via their interactions with astro-particles. Solving quantum Boltzmann equation for GRB-photons, we discuss the generation of circular polarization (as Faraday conversion phase-shift ∆φFC) of GRBs in the following cases: the scattering of GRBs in cosmic microwave background (CMB) and in cosmic neutrino background (CNB) as well as the Compton scattering of GRBs in the presence of Non-Commutative space time and in the galaxy cluster magnetic field.

سمینارهای هفتگی گروه کیهان شناسی

سمینارهای هفتگی گروه کیهان شناسی

درباره ترمودینامیک سیاهچاله ها در دمای صفر

(On black hole thermodynamics at zero temperature)

کمال حاجیان (پژوهشکده فیزیک پژوهشگاه دانش های بنیادی IPM- )

Abstract:

Black holes are some solutions to gravitational theories, which have an event horizon. They behave as thermodynamic systems, e.g. have temperature and entropy, and satisfy laws of thermodynamics. At zero temperature, their entropy does not generically vanish. It makes studying their thermodynamics at zero temperature, a reach subject. I will present some results of our works arXiv:1310.3727 and arXiv:1407.1992, which enhance the black hole thermodynamic laws, at zero temperature.

یکشنبه 19 مهر ماه 1394، ساعت 15:00

دانشکده فیزیک، طبقه اول کلاس فیزیک

By Portal Administrator
10/10/15
116390 Views

سمینار گروه چگال ماده نرم

سمینار گروه چگال ماده نرم

Computational Protein Thermostability Engineering

Hassan Pezeshki Modarres

University of California , Berkeley

شنبه 18 مهر ماه 1394، ساعت 15:30

آمفی تئاتر مرکزی دانشکده فیزیک

By Portal Administrator
10/7/15
116492 Views

سمینارگروه ماده چگال نرم

سمینارگروه ماده چگال نرم

How Age Shapes Anomalous

Diffusion

hadise safdari

یکشنبه 11 مهر ماه 1394، ساعت 15:30

آمفی تئاتر مرکزی دانشکده فیزیک

By Portal Administrator
10/3/15
116608 Views

سمینارهای هفتگی گروه کیهان شناسی

سمینارهای هفتگی گروه کیهان شناسی

بررسی انتقادی قضیه واینبرگ

(Critical review of Weinberg's theorem)

محمد آخشیک (دانشکده فیزیک دانشگاه صنعتی شریف )

Abstract:

Weinberg's theorem is one of the essential elements of linear Cosmological perturbation theory. According to this theorem, "any" cosmological model with FLRW background have two adiabatic perturbative modes which are conserved outside the horizon. Surprisingly, there are important inflationary models that violate this theorem.

In my talk, based on our recent work [arXiv:1508.03293 [hep-th]], after reviewing Cosmological perturbation theory and Weinberg's theorem, I will uncover some hidden assumptions of that theorem and show how some inflationary models successfully evade these assumptions and therefore violate Weinberg's theorem.

سمینارهای هفتگی گروه کیهان شناسی

سمینارهای هفتگی گروه کیهان شناسی

کهکشانهای لایه ای به عنوان آزمون فیزیک بنیادی

(Shell Galaxies as a test of fundamental physics)

هاجر وکیلی (دانشکده فیزیک دانشگاه صنعتی شریف )

Abstract:

Shell galaxies are elliptical ones surrounded by arc-like structures made of stars. They were observed in 1966 for the first time and recently with observation developments became noticeable again. After more than thirty years of study on their origin, now the shells are believed to be consequence of a merger of the elliptical with a dwarf galaxy.

In this talk I will describe how the shell patterns can reveal the underlying physics and use them to verify if it is possible to distinguish between dark matter scenarios and modified gravity models in galactic scales using shell galaxy observations. Modified gravity (MOG), presented by Moffat in 2006 and Modified Newtonian Dynamics (MOND), invented in 1983 by Milgrom are investigated in this context, as well as Cold Dark Matter (with NFW halo) model.

سمینارویژه گروه کیهان شناسی

Abstract: Magnetars are exceptional neutron stars with the highest magnetic fields ( 10^15 gauss) in the universe, an unusual quasi steady X radiation (10^35 ergs/sec) and also produce flares which are some of the brightest events (10^46 ergs in one fifth of a second) to be recorded. There is no satisfactory model of magnetars. The talk will cover neutron stars and a new model for the origin of the magnetic fields in which magnetars arise from a high baryon density ( phase transition) magnetized core which forms when they are born. The core magnetic field is initially shielded by the ambient high conductivity plasma. With time the shielding currents dissipate transporting the core field out, first to the crust and then breaking through the crust to the surface of the star. Recent observations provide support for this model which accounts for several properties of magnetars and also enables us to identify new magnetars.